A 64-year-old man who spent winters living under a Gowanus Expressway overpass apparently froze to death on his foam mattress, as the new year began with what an advocate called a record number of homeless in the city.

The issue could become a test for Mayor Bloomberg, who has yet to say how he intends to deal with the problem.

George Dalton was found Wednesday night in an area about 8 feet high where 86th Street passes over the highway in Bay Ridge.

Dalton wintered there, and told Department of Transportation workers who spoke to him that he was a Vietnam vet.

Police sources said the cause of death is believed to be hypothermia. But Ellen Borakove, spokeswoman for the Medical Examiner’s Office, said an autopsy was inconclusive and more tests are needed.

Mary Brosnahan Sullivan, executive director of the Coalition for the Homeless, called Dalton’s death “shocking and horrible,” and said the homeless problem “could not be more of a crisis.”

“We’re looking at a record number of homeless people in the city,” she said. “Last year at this time, we had a little over 25,000 [men, women and children] in the municipal shelter system each night.

“Now, as we begin 2002, we’re approaching the 30,000 mark,” she said, adding that there are “thousands more on the street.”

Sullivan said her “biggest hope” is that Bloomberg and Police Commissioner Ray Kelly will “craft a safety plan for those shelters that the city directly operates . . . so that homeless people in the street are not fearful of their lives when they go get a cot for the night.”

Bloomberg has said that people shouldn’t sleep on the street, and hopes that solutions to the homeless problem can be found outside the courts.

In a speech today, Kelly says he plans to continue the police focus on quality of life issues.

“We will also assert the public’s right to be left alone – the right not to be pestered by aggressive panhandlers,” Kelly says.

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Do you think the city should force the homeless into shelters? Vote on The Post Web site (www.nypost.com) and sound off on how Mayor Bloomberg should handle the growing homeless problem.

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Aides to Mayor Bloomberg – dissatisfied with the pool of applicants to lead the homeless-services agency – have recruited former Mayor Rudy Giuliani’s commissioner for a brief and highly unusual return stint, The Post has learned.

Sources said that Martin Osterreich, who resigned last month, agreed to a four- to five-week return engagement to give Bloomberg’s aides time to expand an intense search.

“This is an agency that’s really important,” said one source. Sources said Osterreich was brought back because the interim commissioner, Thomas Rozinski, “wasn’t working out.”

In a letter to The New York Times Tuesday, Rozinski vigorously defended city shelters, which homeless-advocacy activists have criticized as unsafe.

Osterreich won’t be able to hang around for long on the job.

After his departure from city government, Osterreich became executive director of the Liberal Party, which will be fielding candidates in at least four special elections over the next few weeks.